1. Circuits and Electrical Current

2. Essential Questions for Circuits
What are the requirements of a circuit?
What is actually going on in a working circuit?
What variable(s) affect what other variable(s) in a circuit? What law(s) and formulas govern the relationship(s) between these variables?
What are the different kinds of circuits? How do they differ (design and use)?

3. Incandescent bulb:

4. How do you make a circuit?
You will be given one bulb, one battery and more than enough wires.
Construct a sketch of how wires can be connected to light a bulb.
Test your design. If it doesn’t work, try variations.
Record your working design(s)
Summarize the requirements of a functioning circuit.

5. The Requirements of a Circuit

6. The Requirements of a Circuit

7. What do you already know?

8. The Basic Function of a Circuit

9. What is “really” happening in the circuit?
Go onto Phet, Circuit Construction Kit (DC Only) Virtual Lab
Build a circuit with one bulb, one battery, one switch, and as many wires as you need to make the circuit show you clearly what is happening when the switch is closed.
What do the moving balls represent? Which way do they move?

10. What is “really” happening in the circuit?
The balls represent electrons – obviously NOT shown to scale.
The electrons move from negative on the battery, around to positive.

11. A Useful Analogy
Sometimes, a closed conducting loop (circuit) is compared to a water (amusement) park. Complete this analogy: Water Park Circuit pump electrons pipes bulb

12. A Useful Analogy
Sometimes, a closed conducting loop (circuit) is compared to a water (amusement) park. Complete this analogy: Water Park Circuit pump battery water electrons pipes wires slide/obstacles bulb

13. Practice
Answer CYU #1,2 on PCR Lesson 2b.

14. Electric Potential Difference
What is it?
What are its units?
How do we measure it?
Is it “the cause” or “the effect” in a circuit?
Read 1b,c
Start your word web

15. Electric Potential Difference
Slang: VOLTAGE
SI unit: volt (V) named after Alessandro Giuseppe Antonio Anastasio Volta
Measured with a voltmeter
Analogous to difference in height, if you want to relate to gravity, or pressure in water pump
By CONVENTION, the positive side of a battery is higher potential.

16. Electric Current What is it? What are its units? How do we measure it?
What variables affect how much current exists in an appliance?
Read 2c
Start your word web

17. Electric Current The flow rate of charges
Unit: ampere (amps, or just A)
1 ampere = 1coulomb of charge flowing thru a conductor in 1 second
Measured with an ammeter
By convention, the “current” flows from positive around to negative.

18. What Variable Affects what other Variable(s) in a Circuit?
What is the mathematical relationship between the ______________(_____) through a bulb and the ______________(___) across the bulb in an electrical circuit?

19. Circuit Schematics

20. What is the mathematical relationship between the current(amps) thru one bulb and the potential difference(volts) across the bulb? What is the mathematical relationship between the current(amps) thru two bulbs and the potential difference(volts) across BOTH bulbs?

21. Understanding the Lab
What happened when potential difference was doubled?
How did the results differ when a second bulb was in the loop?
What do the bulbs do to the current? (more bulbs in series means ____ current)

22. Ohm’s “Law”: I = DV/R
The current in a circuit element (like a bulb) is directly proportional to the potential difference across the element, and inversely proportional to the resistance of the element

23. R: Resistance
R = DV / I
R = rL/A (r=resistivity, L=length, A=cross sectional area) Note: Important in AP E&M
SI unit: ohm (W)
Measured with an ohmmeter
All real conductors have a little of this, insulators have a LOT of it. An ideal conductor has 0W and an ideal insulator has ∞W

24. Resistance What is it? (use The Lazy Mile at the water park)
What happens in the circuit as the resistance changes?

25. Example Problem
Ex: A single loop circuit is made with a 1.5 volt battery, and a light bulb. An ammeter is used to measure a 0.15A current in the loop. What is the resistance of the bulb in this circuit?
PCR Current Electricity 3c CYU 1-5, 7,8

26. (AP only) Checkpoint 3, plus…
…also rank them greatest to least according to: - current density - drift velocity - electric field (strength) in the conductor

27. Lab, Part II
Now, do the lab again, this time with a real bulb, using real wires, and a “battery eliminator.” Only go up to 6.0V
What is different when you graph your data?
Why do you think it is different?
What happened to the brightness of the bulb as you increased the potential difference through it?
What MEASUREMENT relates most closely to brightness in the bulbs?

28. What measurement looks at…
How much charge flows thru a wire in a second?
How the wire prevents the charges to flow?
The difference in energy (carried by each charge) between two positions in a circuit?
The rate at which energy is transferred (or WORK is done)?

29. Measurements in a circuit
I: Current (in amperes (A))
DV: Potential difference (in volts (V))
R: Resistance (in ohms (W))
Power (in watts (W))

30. Equations
I=DV/R DV = IR R = DV/I
P = I(DV) P=I2R P = (DV)2/R

31. Series Circuit

32. Series Circuits
All elements in a series circuit (or branch) have the same current
If you add up the potential differences in the elements that are in a series circuit, the result is equal to the potential difference of the battery.
The power supplied by the battery equals the sum of the power dissipated by all resistors.
In Series, Req = R1 + R2 +…+Rn

33. Example
Draw a series circuit with a 3.0V battery, a 6W light bulb, and a 3W light bulb. Determine the equivalent resistance of the circuit, the current through each item in the circuit, the potential difference across each item in the circuit, the power supplied by the battery, and the power “used” by each of the bulbs.

34. Series Circuits…
Charge is neither created nor destroyed, so the __________ is the same for all parts of the circuit.
Energy is neither created nor destroyed, so the sum of the ______________ for all the bulbs must equal the same measurement for the battery.

35. Parallel Circuit

36. Parallel Circuits
All elements in a parallel circuit have the same potential difference
If you add up the current in all the elements that are in parallel, the result is equal to the current through the battery.
The power supplied by the battery equals the power dissipated by the resistors in total
In Parallel, 1/Req = 1/R1 + 1/R2 +…+1/Rn

37. Parallel Circuit
Draw a circuit with a 3.0V battery, a 6W light bulb, and a 3W light bulb connected in parallel. Determine the equivalent resistance of the circuit, the current through each item in the circuit (that is three different measurements), the potential difference across each item in the circuit, the power supplied by the battery, and the power “used” by each of the bulbs.

38. Parallel Circuits…
Charge is neither created nor destroyed, so the sum of the __________ for all the bulbs is the same as this measurement for the battery.
Energy is neither created nor destroyed, so the sum of the ______________ for all the bulbs must equal the same measurement for the battery.
Each bulb has the same _________ as the battery, because the wires connect each bulb to the battery without any resistance (actually, with negligible resistance).

39. Is Physics Real?
Objective: Determine if the statements made about electric potential difference on slides 32 & 36 are true in actual circuits. If measurements do not show these statements are 100% accurate, why not? Are the statements false, or is something else (another measurement) affecting the investigation.

40. Validating One Rule of Parallel and Series Circuits
Reports: - Objectives - Circuit Diagram, labeled - Data shown neatly - Each statement, followed by the measurement(s) and brief description(s) that support, or refute the claim. If the claim is not (well) supported, explain why you still believe it is true.

41. Practice Problem
A 1.0W and a 2.0W light bulb are wired in parallel, then connected to a 6.0V battery. Determine the power supplied by the battery, and dissipated by each bulb. Which is brightest?
Now, the bulbs are wired in series to the same battery. Determine the power supplied by the battery, and dissipated by each bulb. Which is brightest?

42. Now, we look at the “other”, relevant measurement in electrical circuits…

43. What does the 60 watts mean for a bulb?
A) The bulb has 60 W and after that, it is used up
B) When turned on, the bulb uses 60 watts regardless of the circuit it is in
C) If the bulb is across a potential difference of 120V, the bulb dissipates 60 J of energy each second.

44. 60W vs 120W
Determine the resistance of a “60W” bulb and of a “120W” bulb.
Which draws more current?
What is the cost of running each for 2 hours?

45. What is a KILOWATT-HOUR?
A) a unit of power
B) a unit of electricity
C) a unit of energy
D) a unit of work
E) a unit of current
F) a unit of potential difference
G) the amount of heat needed to warm 1.0 ml of water by one degree Celsius

46. The amount of electrical energy your parents pay about $0.20 for
A kWhr is….
(1000W) x (1hr)
(1000J/s) x (3600s) J
3.6 x 106 J
3.6 MJ
The amount of electrical energy your parents pay about $0.20 for

47. Energy SI unit: joule (J)
Other units: calorie, kilocalorie, kilowatt-hour
E = P * Dt (because a J/s * s = J)
This is the physics measurement most closely analogous to $$$ (1.0kWhr –or 3.6 million joules - of electrical energy costs about $0.25)

48. What does that 60W bulb cost to use for two hours?
(assuming $0.20 for each kWhr)

49. Save some $$$
A 20W CFL provides the same amount of light as a conventional 75W bulb. If the lamp you wish to use is on for two hours a day, how much will the CFL bulb save you in one year? (use $0.20 per kWhr)

50. Apply what you have learned
Build a series circuit with three bulbs. Gradually turn up the potential difference applied across the circuit. Identify the brightest and dimmest bulbs.
Build a parallel circuit with three bulbs. Starting at 1.5V, gradually turn up the potential difference applied across the circuit. Identify the brightest and dimmest.
Rank the bulbs by resistance – record on whiteboard. You MUST JUSTIFY your conclusion with facts about electrical measurements in series and parallel circuits! Compare your results with other groups
Repeat steps 1, but now use meters to measure current and potential difference so you can compute the resistance of each bulb. You will use the handout to organize your measurements, and perform calculations with an excel document. The excel document will also calculate energy each bulb would use in one hour. Were you correct in step 3?

51. AP Performance Assessment
Objective: Determine the EMF and internal resistance for two different D-cell batteries.
Report:
Title/Objective
Circuit Diagram
Data
Computations (4 steps)
Conclusion (be sure the numerical answers are part of this!)